A contact charging method which comprises applying a voltage while pressing against the surface of a material to be charged such as photoreceptor to electrostatically charge the surface of the material to be charged is advantageous in that the required applied voltage is lower than that in corona charging method. As such an electrostatic charging member there has been widely used an electrically-conductive elastic material layer or an electrically-conductive material-coated roll, belt or blade. In order to render these electrostatic charging members electrically-conductive, a high molecular material such as rubber and resin comprises carbon black, a metal oxide or an ionically-conducting agent dispersed therein.
However, it is extremely difficult to obtain a solution having an electrically-conducting agent such as carbon black and metal oxide uniformly dispersed in a high molecular material. The coating layer formed by the insufficiently-dispersed solution has a high resistance moiety and a low resistance moiety. The resulting electrostatic charging member has a nonuniform resistance. Thus, image defects such as black streak and blank area attributed to uneven charging can occur. Further, leakage can occur between the low resistance moiety and pinholes present in the material to be charged, similarly causing image defects such as black streak.
For countermeasure against these troubles, an action such as incorporation of a dispersing aid in a high molecular material has been studied. For example, JP-A-5-134515 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes an electrostatic charging material comprising an electrically-conductive material and a material having a chemical interaction with the electrically-conductive material as a dispersing aid for the electrically-conductive material dispersed in an elastic matrix polymer.
However, the foregoing method comprising the incorporation of a dispersing aid is disadvantageous in that the uniformity in dispersion of electrically-conducting agent is not always sufficient and the dispersing aid or its unreacted components separate out from the high molecular material as bleedout, causing tack with the material to be charged. The foregoing method is also disadvantageous in that the bleedout of dispersing aid contaminates the material to be charged, resulting in the occurrence of image defects such as black streak and blank area.
Further, the ionically-conducting agent is excellent in resistance uniformity but is disadvantageous in that when kept in contact with the material to be charged over an extended period of time, ionic materials can migrate to the material to be charged, similarly causing image defects such as black streak and blank area.